Charging device of a secondary battery for controlling termination of charging in response to the change state of terminal voltage of the secondary battery

Abstract

A control section detects terminal voltage of a battery periodically and stores it in a memory, then calculates the most recent gradient and average gradient of a charging curve from the storage contents. When the most recent gradient becomes greater than the average gradient by a predetermined value or more, the control section judges it to be the last stage of the charging. After this, the control section compares the most recent terminal voltage with the preceding terminal voltage which are detected and stored in sequence. When they become equal to each other, the control section turns off a charge switch for terminating the charging. Thus, a rise in the terminal voltage at the last stage of the charging is detected securely and the subsequent peak is detected accurately.

What is claimed is: 
     
       1. A charging device of a secondary battery for detecting terminal voltage of the secondary battery during charging for controlling termination of a charging operation, said charging device comprising: means for supplying a charging current to the secondary battery;   means for detecting a terminal voltage of said secondary battery;   most recent gradient calculation means for finding a most recent gradient of the terminal voltage detected by said detection means;   average gradient calculation means for finding an average gradient, for a comparatively long term, of the terminal voltage detected by said detection means;   gradient comparison means for comparing said most recent gradient with said average gradient;   voltage comparison means for comparing the most recent terminal voltage with a previously measured terminal voltage;   termination determination means responsive to outputs of said gradient comparison means and said voltage comparison means for determining termination of charging by the fact that said most recent terminal voltage becomes equal to or less than said previously measured terminal voltage at a time after said most recent gradient becomes greater than said average gradient by at least a predetermined value; and   control means responsive to said termination determination means for controlling said charging current supply means so as to stop the charging current to the secondary battery.   
     
     
       2. The charging device as claimed in claim 1 further including means for storing terminal voltage data of the secondary battery detected by said voltage detection means. 
     
     
       3. The charging device as claimed in claim 2 wherein said storage means stores a predetermined number of voltage data pieces including the most recent data piece detected by said voltage detection means, wherein storage of the most recent voltage data piece causes said storage means to discard the oldest voltage data piece. 
     
     
       4. The charging device as claimed in claim 3 wherein said voltage detection means detects the terminal voltage repeatedly at given time intervals. 
     
     
       5. The charging device as claimed in claim 4 wherein said most recent gradient calculation means calculates repeatedly the most recent gradient at the same given time intervals as the given time intervals at which said voltage detection means detects the terminal voltage by using two terminal voltages detected at time intervals each being an integer multiple of said given time interval. 
     
     
       6. The charging device as claimed in claim 5 wherein said two terminal voltages used to calculate the most recent gradient are the most recent and oldest ones of a predetermined number of voltage data pieces stored in said storage means. 
     
     
       7. The charging device as claimed in claim 4 wherein said voltage comparison means makes repeatedly a comparison at the same given time intervals as the given time intervals at which said voltage detection means detects the terminal voltage by using two terminal voltages detected at time intervals each being an integer multiple of said given time interval. 
     
     
       8. The charging device as claimed in claim 7 wherein said two terminal voltages used to make a comparison are the most recent and oldest ones of a predetermined number of voltage data pieces stored in said storage means. 
     
     
       9. The charging device as claimed in claim 4 wherein said most recent gradient calculation means calculates repeatedly the most recent gradient at the same given time intervals as the given time intervals at which said voltage detection means detects the terminal voltage by using two terminal voltages detected at time intervals each being an integer multiple of said given time interval; and   said voltage comparison means makes repeatedly a comparison at the same given time intervals as said most recent gradient calculation means calculates the most recent gradient, wherein the two terminal voltages to be compared with each other are the same as the two terminal voltages used for the said most recent gradient calculation means to calculate the most recent gradient.   
     
     
       10. The charging device as claimed in claim 9 wherein the two voltage data pieces used to calculate the most recent gradient by said most recent gradient calculation means and to make a comparison by said voltage comparison means are the most recent and oldest voltage data pieces stored in said storage means. 
     
     
       11. The charging device as claimed In claim 4 wherein said average gradient calculation means calculates a difference between the most recent and oldest voltage data pieces stored in said storage means, accumulates the resultant difference, and divides accumulative value by the number of times the difference has been accumulated for calculating an average gradient. 
     
     
       12. The charging device as claimed in claim 11 wherein said difference between the most recent and oldest voltage data pieces is accumulated from starting of charging to the present. 
     
     
       13. The charging device as claimed in claim 4 wherein said charging termination determination means turns on an identification flag when it is determined that the most recent gradient becomes greater than the average gradient by the predetermined value or more in response to the output of said gradient comparison means, said termination determination means, when the flag is on, for monitoring the output of said voltage comparison means and determining termination of charging. 
     
     
       14. The charging device as claimed in claim 1 wherein said secondary battery is a nickel-hydrogen battery. 
     
     
       15. A method of charging a secondary battery for detecting terminal voltage of the secondary battery during charging for controlling termination of the charging, said charging method comprising: a step for detecting a terminal voltage of said secondary battery;   a most recent gradient calculation step for finding a most recent gradient of the terminal voltage detected by said detection step;   an average gradient calculation step for finding an average gradient, for a comparatively long term, of the terminal voltage detected by said detection step;   a gradient comparison step for comparing said most recent gradient with said average gradient;   a voltage comparison step for comparing the most recent terminal voltage with a previously measured terminal voltage;   a termination determination step subsequent to said gradient comparison step and said voltage comparison step for determining termination of charging by the fact that said most recent terminal voltage becomes equal to or less than said previously measured terminal voltage at a time after said most recent gradient becomes greater than said average gradient by at least a predetermined value; and   a control step responsive to the termination of charging determined by said termination determination step for terminating the charging current to the secondary battery.   
     
     
       16. A method of charging a secondary battery comprising: detecting a terminal voltage of said secondary battery;   calculating a most recent gradient of the terminal voltage detected by said detecting step;   calculating an average gradient over a comparatively long term, of the terminal voltage detected by said detecting step;   comparing said most recent gradient with said average gradient;   comparing the most recent terminal voltage with a previously measured terminal voltage subsequent to said gradient comparison step and generating a termination signal in response to said most recent terminal voltage becoming equal to or less than said previously measured terminal voltage at a time after said most recent gradient becomes greater than said average gradient by at least a predetermined value; and   terminating the flow of charging current to the secondary battery in response to said termination signal.